Steam Condensate

James Hall, PE

Please comment on the following recommendation from a steam expert:

Normally, in an application such as your the steam condensate from the two heat exchangers is piped into a dilution tank where it is mixed with cold city water and then sent down the drain. However, this is not the case in your application. It was observed that the steam condensate is being mixed with cold city water in the condensate drainpipe itself. This condition could cause the condensate not to drain freely and cause it to back up in the heat exchanger as is happening in this case.

Any help would be appreciated.
James Hall, PE

David Efflandt

depends upon pipe config

While my only steam experience is my home boiler, I do have a BSME and work with automatic valves on systems with tanks and pumps. Whether the cold water helps or hurts the flow of water from the heat exchangers depends upon how the cold water is piped into the condensate line and velocities. For example if the condensate flows straight through a T and the cold water comes in the side, it could hit the far side and somewhat push upstream and downstream. Or if both come in the sides of the T and out the middle, the cold could fight the hot.

In one case I saw a system with 2 pumps pumping out of a tank, and due to piping configuration, 1 pump could pump more water than when both pumps were running.

In other configurations like a Y or ejector/eductor, the cold water pressure and velocity could help the flow of condensate. We have ejectors on some of our pilot control systems that can use a pressure source to draw a vacuum (kind of like devices that can go on a faucet to empty a waterbed).

Jerry Boulanger_2

Most condensate piping

is selected based on gravity flow, and the capability of a pipe will depend on its size and slope. I think what your expert is trying to say is that by injecting the cold city water into the condensate pipe, the total flow in the pipe (city water plus condensate) is too high and the condensate is backing up. The 2001 ASHRAE Fundamentals Handbook Chapter 35 has information on the capacity of condensate pipes in various types of installations.

Tony Conner

It's...

...entirely possible that this guy is accurately describing what is happening. The cooling water flow, combined with the condensate could be enough to overwhelm the drain line. This could mean a pressure increase on the downstream side of the trap, backing condensate up into the heat exchangers. What is the condensate pipe material at and downstream of the cooling water connection? If it's carbon steel, you may be getting a LOT of corrosion in this line, choking it off and contributing to the problem.

How was the cooling water supply sized? Is it fed through a temp control valve? If it's there, is it set properly?